Autism spectrum disorder (ASD) is a major public health problem affecting 1 out 110 children. There is a fundamental gap in understanding the cellular and molecular mechanisms underlying the diverse clinical presentation of ASD. Understanding these mechanisms is critical for developing novel therapeutic approaches. The limitations inherent in human studies make it difficult to study cellular and molecular mechanisms, providing a rationale for the study of animal models. Recent genetic evidence implicates the SHANK3 gene in ASD. SHANK3 is a scaffolding protein that organizes a signaling complex at postsynaptic density of excitatory synapses. An array of SHANK3 isoforms result from 6 alternative promoters and splicing of coding exons. Deletion of the SHANK3 gene is a major contributor to ASD features in the 22q13.3 deletion Phelan-McDermid syndrome. Microdeletions of the entire SHANK3 gene and point mutations of SHANK3 disrupting specific isoforms have been identified in patients with ASD and intellectual disability. We and others reported Shank3 isoform-knockout mice with different exonic deletions. Reduced postsynaptic response of excitatory synapses in hippocampus, striatum, and neocortex as well as ASD-like behaviors is found in these mice, but there were also notable phenotypic differences. This phenotypic heterogeneity could be explained by the different impact that each mutation has on isoform-specific expression of Shank3. However, interpretation of these differences is complicated by the fact that the existing mutant mice are not Shank3 complete knockout and were analyzed in different brain regions and using different protocols. Because >95% of SHANK3 molecular defects in humans delete the entire SHANK3 gene, and these patients have more severe phenotypes than those with point mutations of SHANK3, we have generated Shank3 complete knockout mice by deleting exons 4-22, and also Shank3 conditional knockout mice, with floxed exons 4-22. These models are more valid for dissecting the cellular mechanism arising from SHANK3 deletion than existing isoform-knockout mice. The objective of this proposal is to analyze Shank3 complete knockout mice using an interdisciplinary approach. The central hypothesis is that the primary path by which SHANK3 molecular defects lead to ASD in humans is via cellular and synaptic defects of reduced glutamatergic receptor-mediated postsynaptic responses in different brain regions. With these mutant mice, we are uniquely positioned to study the following Specific Aims: 1) To model major clinical features of SHANK3 deletion in Shank3 complete knockout mice; 2). To delineate the cellular and molecular mechanism underlying Shank3 deficiency; 3) To analyze the temporal- and spatial-requirements of SHANK3 deficiency underlying the pathogenesis of ASD. The proposed studies are significant because analysis of Shank3 complete knockout mice may uncover the cellular and circuit mechanism of ASD caused by SHANK3 deletion. The knowledge gained from studying Shank3 will provide insights that may be generalizable and applicable to understanding the pathophysiology of other causes of ASD.